Method of forming an electrical heating element



July 11, 1967 s. E.-PRICE 3,330,034

METHOD OF FORMING AN ELECTRICAL HEATING ELEMENT Filed April 13, 1962 2 Sheets-Sheet 1 v INVENTOR GEORGE E. PRICE ATTORNEY J ly 7 s. E. PRICE 3,330,034

METHOD OF FORMING AN ELECTRICAL HEATING ELEMENT Filed April 13, 1962 2 Sheets-Sheet 2 INVENTOR GEORGE E.PRICE WKYFM v TTORNEY United States Patent 3,330,034 METHOD OF FORMING AN ELECTRICAL HEATING ELEMENT George E. Price, Mansfield, Ohio, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 13, 1962, Ser. No. 187,417 6 Claims. (Cl. 29-611) This invention relates to formation of sheathed electrical heating elements, concerning especially cross-sectional deformation thereof.

A well known type of electrical heating element has an internal high-resistance electrical conductor embedded in heat-conductive, electrically insulating, refractory material inside a protective sheath, preferably of metal capable of being Worked to permit compaction of the refractory material, which usually is in granular or similar finely divided form. Such heating elements are used in cooking ranges, hot-water heaters, and similar appliances.

The trend is toward increasing the heating capabilities of each size of such heater by raising the watts density imposed on the resistance wire and on the surrounding sheath. Despite increased care in their manufacture, such heating elements may fail prematurely because of the excessive demands made upon them, especially in the event (not uncommon) of excessive line voltage. Many or most of such failures can be traced to hot-spotting of the resistance heating element at locations of insufficiently dense refractory material and consequently deficient heat-conductivity from the wire to the sheath,

A primary object of the present invention is increased useful life of sheathed electrical resistance heating elements.

An object is elimination of insufficiently dense regions of refractory material surrounding the resistance wire in such electrical heating elements.

A further object is improvement in the operational watts density of an electrical heating element of the kind described.

Other objects of this invention, together with means and methods for attaining the various objects, will be apparent from the following description and the accompanying drawings:

FIG. 1 is a side elevation, partly in section, of a sheathed electrical heating element at one stage in its formation;

FIG. 2 is an enlarged transverse cross-section of the heating element of FIG. 1, taken at 11-11 thereon;

FIG. 3 is a side elevation, partly in section, of the same heating element at a later stage in formation, according to this invention;

FIG. 4 is a side elevation, partly in section, of the heating element of the preceding views, at a later stage in its formation;

FIG. 5 is an enlarged transverse cross-section of the heating element, taken at V-V on FIG. 4; and

FIG. 6 is another enlarged transverse cross-section, taken at VI-VI on FIG. 5.

FIG. 7 is a side elevation, partly in section, of an electrical heating element like that of FIG. 1, but at a later stage in its formation and on a somewhat smaller scale;

FIG. 8 is an enlarged transverse cross-section of the heating element of FIG. 7, taken at VIIIVD1 thereon;

FIG. 9 is a side elevation, partly cut away, showing formation of the heating element of FIG. 3 from that of FIG. 1; and

FIG. 10 is a side elevation, partly cut away, showing formation of the heating element of FIG. 4.

In general, the objects of the present invention are accomplished, in formation of a sheathed electrical heat- Patented July 1 1, 1967 ing element containing an electrical conductor embedded in heat-conductive, electrically insulating, refractory material and provided with an electrically conducting terminal aflixed at its inner end to the conductor and protruding at its outer end from the sheath, by compressing the sheath laterally to reduce its cross-sectional area about the location of the inner end of the terminal without elongating it or with a minimum amount of elongation, after having elongated it, as by previous lateral compression, in that location. The invention contemplates particularly that the non-elongating lateral compression about the location of the inner end of the terminal will provide the final change in cross-section of the sheath in the vicinity thereof or of the entire sheath.

FIG. 1 shows, in side elevation, with the right half thereof sectioned along a longitudinal medial plane, an electrical heating element at a preliminary stage in its formation according to this invention. A cylindrical sheath 11 contains an electrical conductor 13, which takes the form of a helix concentric with the longitudinal axis of the sheath. Prot-ruding axially from opposite ends of the sheath are a pair of terminals 15 and 16, at the left and right, respectively. An annular plug 18 is visible, surrounding the terminal 16, just inside the right end of the sheath. The inner end of the right terminal is covered by contiguous turns of the helical conductor, all these components being concentric with one another. Off the end of the terminal the turns of the helix are spaced from, instead of contiguous with, one another. The interior of the left half, hidden by the sheath (which is sectioned away at the right half of this view), is similar. The entire interior of the sheathis filled with refractory material 19, which is shown in granular or similar finely divided solid form. FIG. 2 shows, on an enlarged scale, the same heating element in transverse section (i.e., perpendicular to the longitudinal axis), as indicated at ]1-II on the preceding view.

FIG. 3 shows, in side elevation, partly in section, this heating element at a later stage in its formation according to this invention. Although the corresponding parts are denoted by unchanged reference numerals, and although the drawing is not to scale, it is apparent from this view that the heating element is elongated here, as compared with its appearance in FIG. 1. The diameter of the sheath is diminished, as is the cross-sectional area. Except on the end of the terminal, where the turns of the helical conductor may or may not remain contiguous, the spacing of the turns is increased and the helix diameter is decreased. While hardly apparent from this show ing, the refractory material is more compact that it was in the initial condition of the heating element, shown in FIG. 1.

FIG. 4 shows, in like manner, the same electrical heating element after further formation according to this invention. Near each end, the portion of the sheath about the inner end of the terminal at that location is reduced further in cross-section and (as shown articularly in subsequent views) is primarily hexagonal, rather than circular, in transverse outline. Reduced portions 25 and 26 at the left and right, respectively, surround the inner ends of terminals 15 and 16 and extend each way for a limited distance in the axial or longitudinal direction from the actual ends, which themselves (although hidden) are denoted by the conversion from contiguous to spaced turns of the helical conductor. Each of these reduced portions of the sheath is hexagonal over the major part of its length and has narrow tapered transition bands joining the hexagonal part to the adjacent circular sheath. Transition bands 21 and 22 flank portion 25, while like bands 23 and 24 flank portion 26.

3 FIG. 5 shows, on an enlarged scale, hexagonal'reduced portion 26 sectioned transversely through the inner end portion of terminal 16, as indicated on the preceding view. FIG. 6 is a like section taken, as indicated on FIG. 4, to the left of (i.e., off) the inner end of the terminal. Here helical conductor 13 itself is hexagonal in outline (of V the helix) because, without the terminal inside to support it, it has been deformed by the same forces (transmitted to it through the refractory material) that altered the transverse outline of this sheath portion from round to six-sided.

FIG. 7 shows, principally in side elevation but with one end portion in longitudinal section, an electrical heating element, having cylindrical ends located side by side, the longitudinal axis being at least partially curved. An arcuate intermediate portion 28'has a ftattened'side 29, which is parallel to the plane of the paper. As in FIGS. 1, 3 and 4, in all of which the heating element was rectilinear from end to end, part of the intermediate portion is broken away completely as superfluous to the showing. The scale of FIG. 7 is somewhat smaller than the scale of those previous views, whereas FIG. 8, which shows the transverse cross-sectional appearance of the curved and flattened intermediate portion, is on the same scale as FIGS. 2, 5 and 6. This cross-section may be considered D-shaped, withits fi-at side 29 at the top in this view. The electrical conductor 13, as well as the sheath, has such an outline.

FIG. 9 shows, in side elevation, partly cut away and in section, a pair of flanged roller dies 31 and 32 on axles 33 and 34, respectively, with sheath 11 of an electrical heating element of this invention being rolled between them, as shown by the arrows; Reduction in diameter of rolled portion 38, as compared with unrolled portion 37, is apparent. The near flanges of the roller dies are broken away in the vicinity of the work to show the transition from larger to smaller diameter.

FIG. shows in side elevation, partly cut away and in section, a pair of stamping dies 41 and 42 converting a portion (e.g., portion 25) of the sheath of a heating element of this invention from round to hexagonal crosssection, and forming transition bands 21 and 22 flanking the hexagonal part, as in FIG. 4.

Although deformation of the sheath of an electrical heating element of the kind described to reduce the crosssectional area thereof and thereby compact the finely divided refractory material inside is well known, the present invention performs a plurality of compacting steps in a certain sequence in order to bring about the desired results. With this principle in'mind, the practice of this invention is readily understood.

A preferably cylindrical (round in transverse section) electrical heating element as shown in FIG. 1 is compressed transversely to reduce it in cross-section and to elongate it. Some degree of permanent elongation is almost inevitable in compressing the sheath'over all or nearly all of its'peripheral'surface, and when itis rolled, as in FIG, 9, the resulting length may represent an increase of one-tenth to one-half (or even more) over the original length. A general compaction of the refractory material occurs during this" step, increasing the heat-conducting capabilitythereof, which is important to prevent overhe ating of theelectrical conductor.

The terminals joined to the electrical conductor, which itself usually is in helical'form as shown, are retained finnly in the annular plugs that fill the end of the sheath. The portions of these terminals located inside the sheath are subjected to transverse or lateral force'transmitted through the surrounding refractory material during the elongation step. Because much of the compressive force applied to the exterior of the sheath is, dissipated in compaction of the refractory material and because thevterminals are usually solid, rather than hollow, the terminals themselves elongate less than the sheath does, perhaps no more than one-fourth as much. Accordingly, the inner end' of the terminal is displaced axially away from its original location in the direction of movement of the near end of the sheath and the confining plug. Theoretically,

the terminal pin pulls out of the refractory material or 1 does not elongate with the sheath, leaving a void whose volume is equal to the area of the terminal pin multiplied by the distance the pin pulls out of the refractory material.

Actually, this void is partially filled with refractory material from the surrounding area, but the density thereof in the void is relatively low and at a minimum. A 10- calized minimum density 'of refractory material provides a temperature maximum for the electrical conductor there, with consequent risk of failure of theconductor at that location throughoverheating.

According to the present invention, after the sheath is compressed laterally and elongated, as justdescrib'ed, and preferably after completionof all other distorting steps that might elongate it'further, the portion of the sheath surrounding and adjacent the inner 'end' of each terminal displaced by elongation of the sheath is compressed further to reduce the cross-section without further elongation or with a very minimum amount of elongation.

This final step provides increased compaction of the re-' fractory material about and adjacent the inner end of the terminal, to perhaps an average density of 3.00 to 3.25 grams per cc. (magnesia again), thereby eliminating the objectionable density minimum, as compared with the density prevailing throughout the intermediate (major) part of the heating element.

This further localized deformation of the sheath to re-- 7 duce its cross-sectional area without elongating it mayg be accomplished, with a sheath of round outline,- by imparting to the sheath a non-round (e.g., polygonal) outline having a perimeter equal to the original circular perimeter. This might be accomplished by means of stamping dies, as in FIG. 10, or by any suitable swaging operation. The axial extent of this further compaction should be limited to the distance equivalent to at least about one but preferably not more than about two di-' :ameters (of the sheath). It should be extensive enough:

to encompass the region of minimum density but not so extensive as to be conductive to further elongation of the sheath, which might aggravate the condition-sought i to be remedied. 7

It will be eapparentthat in the step of elongating the sheath originally, as by rolling it, the transverse outline may be (or be converted to) polygonal, for,example, rather than round. If so, the further localized non-elongating deformation about and adjacent-the inner end of the terminal may reduce the number of sides (e.g., from six to three) while retaining the same perimeter or pe- V ripheral length, as by suitably shaped dies.

-Where an intermediate portion of the sheath is'toI-be curved to. some extent,.whether into one or more U-turns or a spiral configuratiom'for example, it is cus tomary to deform'the sheath transversely. or laterally afterward'to compact the refractory material, especially on the outside of the curve or curves, where radial density minima are usual, Such further deformationmay be along one side only, producing a D-shaped outline, as 'shown in. FIG. .8.

Normally such curvature does not begin near or include 7 the inner end of the terminal and, thus, maybe ignored in the practice of this invention. However, in the event that any step in such deformation should elongate/the: sheath in the vicinity of the terminal, the final-localized non-elongating deformation about'and near the innerend of the terminal according to this invention shouldbe .performed thereafter, rather than before (01 again, in ad dition thereto, rather than on1-y'before).' a

The practice of this invention requires no unusual -ma-' terials -of construction internal or. external to the heating element. As is customary, the sheath may be composed of steel, copperyaluminum or nickel alloys, for example, the internal electrical conductor. 10f 'metal having suitably high electrical resistance, such as nichrome, and the terminals of copper or other good conductor. The annular plugs for the end of the sheath may be of suitable rubber, plastic, ceramic, or igneous mineral material deformable to the desired extent, as where the entire length of the sheath is rolled, for example. Initial clearance between the plug body and the sheath or the electrical conductor (or both) may be provided to absorb part of such deformation. The refractory material may be an oxide, a silicate, or mixture of one or more :of either or both, of aluminum or magnesium, for example, with or without a binder, and in suitably compactable form.

The practice of this invention greatly increases the average operating life of electrical heating elements subjected to it. One index of this is an observed increase of more than ten percent in the failure wattage of heating elements so treated, over that of like elements untreated. As normal operating watts density is appreciably less than the failure density, it is clear that the expected increase in life of the heating element of this invention is much more than that percentage. The full benefits and advantages of this invention will become apparent and accrue to those who practice it.

It will be understood that the invention as shown and described is exemplary only, and that departures therefrom or modifications therein may be made without deviating from the invention as claimed.

What is claimed is:

1. In the formation of an electrical heating element having a deformable sheath,

an electrical conductor inside the sheath,

a terminal affixed to the conductor and protruding outside the sheath, and

compactible refractory material filling the interior of the sheath around the conductor and the terminal, wherein the sheath is elongated axially, the improvement comprising subsequently laterally compressing the portion of the sheath in the vicinity of the inner end of the terminal and simultaneously maintaining the length of the compressed portion substantially constant, thereby reducing that sheath portion in cross-section and compacting the refractory material in the vicinity of the inner end of the terminal.

2. In the deformation of an electrical heating element having a tubular sheath containing :an electrical conductor embedded in heat-conductive, electrically insulating, refractory material and terminating in a protruding terminal, to compact the refractory material, the improvement comprising axially lengthening, and then laterally compressing at substantially constant length, the sheath about and near the inner end of the terminal and thereby further reducing the tubular cross-section therea-bout and compactin the refractory material in the vicinity of the inner end of the terminal. 3. Process of forming a sheathed electrical heating element containing 5 an electrical conductor embedded in heat-conductive, electrically insulating, refractory material and provided with an electrically conducting terminal afiixed at its inner end to the electrical conductor and protruding at its outer end from the sheath, comprising laterally compressing and simultaneously axially elongating the sheath, and thereafter laterally compressing at substantially constant length the portion of the sheath in the vicinity of the affixed inner end of the terminal and thereby reducing it in cross section more than the portions of the sheath adjacent thereto.

4. The process of claim 3 wherein the compression at constant length is performed only after completion of all zaxial elongation to which the portion of the sheath surrounding the inner end of the terminal is subjected in the formation of the heating element.

5. Process of forming an electrical heating element having a helical electrical conductor embedded in heat-conductive, electrically insulating, refractory material in compactable form inside and filling a cylindrical metal sheath closed at the ends and having an electrically conducting terminal affixed to the conductor and extending axially of a portion of the sheath to the exterior thereof, comprising the steps of laterally compressing and axially elongating the sheath and thereby displacing the terminal longitudinally, and thereafter laterally compressing the portions of the sheath immediately surrounding the portions of the interior thereof occupied by the terminal before the displacement, to reduce the cross-sectional area of those sheath portions while maintaining their perimeter essentially constant.

6. The process of claim 5 wherein transverse outline of the sheath after completion of the initial step is round, and the final transverse outline of the sheath portions compressed in the step performed thereafter is polygonal.

References Cited UNITED STATES PATENTS 2,428,900 10/1947 Wiegand 29155.64 2,591,422 4/1952 Lacy-Hulbert et al. 29155.64 2,677,172 5/1954 Oakley 29155.64 2,785,270 3/1957 Burger 29155.64 2,846,537 8/1958 Thornhill 29155.65

2,933,805 4/1960 McOrlly 29155.65

JOHN F. CAMPBELL, Primary Examiner. I. M. ROMANCHIK, I. W. BOCK, Primary Examiners. 

1. IN THE FORMATION OF AN ELECTRICAL HEATING ELEMENT HAVING A DEFORMABLE SHEATH, AN ELECTRICAL CONDUCTOR INSIDE THE SHEATH, A TERMINAL AFFIXED TO THE CONDUCTOR AND PROTRUDING OUTSIDE THE SHEATH, AND COMPACTIBLE REFRACTORY MATERIAL FILLING THE INTERIOR OF THE SHEATH AROUND THE CONDUCTOR AND THE TERMINAL, WHEREIN THE SHEATH IS ELONGATED AXIALLY, THE IMPROVEMENT COMPRISING SUBSEQUENTLY LATERALLY COMPRESSING THE PORTION OF THE SHEATH IN THE VICINITY OF THE INNER END OF THE TERMINAL AND SIMULTANEOUSLY MAINTAINING THE LENGTH OF THE COMPRESSED PORTION SUBSTANTIALLY CONSTANT, THEREBY REDUCING THAT SHEATH PORTION IN CROSS-SECTION AND COMPACTING THE REFRACTORY MATERIAL IN THE VICINITY OF THE INNER END OF THE TERMINAL. 